1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, a liquid crystal driver and a method for driving an LCD panel, and in particular a technique to drive the LCD panel by an inversion drive method.
2. Description of the Related Art
The inversion drive is regarded as one of the techniques that are widely used to drive the liquid crystal display panel. The inversion drive is a driving method which inverts the polarities of data signals provided to data lines (or signal lines) at appropriate time and spatial intervals in order to prevent image “burn-in” of the LCD panel. The inversion drive reduces DC components of drive voltages applied to the liquid crystal capacitors within respective pixels, and effectively prevents the image “burn-in” phenomenon.
The inversion drive includes two kinds of methods: a common constant driving method and a common inversion driving method. The common constant drive method involves inverting the polarities of data signals while sustaining the potential level of a common electrode (or an opposite electrode) unchanged; the potential level of the common electrode is referred to as the common potential VCOM, hereinafter. On the other hand, the common inversion drive method is a driving method which inverts both the data signal and the common potential VCOM. The common constant drive method has an advantage of excellent stability in the common potential VCOM compared to the common inversion driving method. As well-known to those skilled in the art, the stability of the common potential VCOM is important in terms of suppressing flickers.
One of the typical common constant driving methods is a dot inversion drive in which the polarities of data signals applied to respective pixels are spatially inverted with respect to both horizontal and vertical directions. It should be noted that the polarities of the data signals are defined with respect to the common electrical potential VCOM in this specification. The dot inversion drive further improves the stability of the common potential VCOM, and effectively suppressing the flickers. Most typically, the spatial interval in which the polarities of the data signals are inverted is one pixel with respect to both the horizontal and vertical directions. However, the dot inversion drive in this specification should be understood as including the case that the spatial interval in which the polarities of data signals are inverted is two or more pixels, and the case that the spatial interval in which the polarities of data signals is inverted is different between the horizontal direction and the vertical direction.
In the dot inversion drive, the potential levels of the data lines are inverted in order to invert the data signals written into the pixels with respect to the vertical direction. The polarities of the potential levels of the data lines when the data signals are written into pixels in a specific horizontal line are opposite to the polarities of the potential levels of the data lines when the data signals are applied to pixels in the adjacent horizontal line.
A problem accompanied by the inversion of the potential level of the data lines is that increased power is required to invert the potential levels of the data lines due to an extremely large capacity of the data lines, which will undesirably cause the increase of power consumption in liquid crystal display devices. The increased power consumption to invert the potential level of the data lines is one of the serious problems, particularly in a liquid crystal display device within a cellular phone terminal.
One approach has been proposed as a technique to suppress the power consumption in the liquid crystal display devices, which involves short-circuiting data lines before inverting the potential levels of the data lines. Japanese Laid-Open Patent Application No. Jp-A Heisei 11-95729, for example, discloses a technique in which adjacent data lines are short-circuited before inverting the potential levels of the data lines within the liquid crystal display device adapted to dot inversion drive with the spatial interval to invert the data signals configured to one pixel. Short-circuiting the data lines effectively allows electric charges accumulated in the data lines to be effectively utilized, and thereby suppresses the power consumption in the liquid crystal display device. Japanese Laid-Open Patent Application No. Jp-A 2002-62855 also discloses a technique in which data lines are not short-circuited in a non-inverting period during which the polarities of potential levels of data lines are not inverted for the further suppressing the power consumption.
Another important factor to suppress the power consumption of the liquid crystal display device is reduction of power consumption in operational amplifiers used for driving data lines.
The techniques disclosed in these patent applications, however, suffer from a problem of useless power consumption in the operational amplifiers. This is because the driving capabilities of the operational amplifiers are not controlled in the disclosed liquid crystal drivers. In an architecture of the liquid crystal drivers in which a pair of data lines are short-circuited before inverting the potential levels of the pair of data lines, the operational amplifiers need to have a sufficient drive capability to charge (or discharge) the respective data lines from an average potential level of the pair of the data lines to the potential levels indicated by the associated pixel data. Accordingly, when the difference between the average potential level of the pair of the above data lines and the potential levels indicated by the pixel data is small, the drive capability of the operational amplifiers should be small; however, the liquid crystal drivers disclosed in the above-mentioned patent applications do not have function of controlling the drive capability of the operational amplifiers. In the conventional techniques, the operational amplifiers are required to be designed with a drive capability to cope with a maximum difference between the average electrical potential of the pair of the data lines and the electrical potentials indicated by the with the pixel data. This undesirably increases power consumption of the operational amplifiers.
With respect to the above-described problem, techniques are disclosed which reduce power consumption of the operational amplifiers by controlling the drive capability and the use/no-use in the operational amplifiers. Japanese Laid-Open Patent Application No. Jp-A Heisei 5-41651, for example, discloses a technique in which a drive capability of each amplifier is controlled in response to a difference between an output signal provided from the operational amplifier and an input signal voltage. In this technique, the drive capabilities of respective operational amplifiers are increased when a difference between the output signal and the input signal voltage is large, and the drive capabilities of the operational amplifiers are decreased for a small difference. Since reduction in the drive capability effectively reduces power consumption of the operational amplifiers, the power consumption of operational amplifiers is suppressed by reducing the driving capabilities of the operational amplifiers when a large drive capability is not required.
Japanese Laid-Open Patent Application No. Jp-A 2004-45839 further discloses a technique to deactivate operational amplifiers in response to pixel data associated with pixels in the horizontal line and pixel data of the corresponding pixels in the adjacent horizontal line. More specifically, this patent application discloses that data lines are driven by D/A converters without using operational amplifiers when the pixel data of all the pixels in the horizontal line are identical to the pixel data of the corresponding pixels in the adjacent horizontal line. When the pixel data of one pixel in a horizontal line is detected as being different from that of the corresponding pixel in the adjacent horizontal line, the operational amplifiers are used to drive the data lines.
However, these techniques do not provide a technique for controlling the drive capability of the operational amplifiers suitable for architecture in which the data lines are short-circuited before driving data lines.